PORTLAND, Maine  A survey of Cobscook Bay has uncovered the presence of sea squirts, an invasive species that scientists fear could overwhelm valuable shellfish beds and alter the marine ecosystem, researchers said Monday.

The survey this month confirmed an initial sighting of Didemnum, a type of sea squirt that has rapidly colonized on Georges Bank fishing ground.

The concern is that sea squirts can quickly overrun the ocean's bottom, covering everything from rocks to scallops and mussels. People describe the thick mats having the appearance of pancake batter on the ocean floor.

"It's an almost scary sort of animal. It smothers everything that's underneath it," said Larry Harris, a professor from the Center for Marine Biology at the University of New Hampshire who participated in the survey.

Sea squirts are tunicates, a filter-feeding sea creature named for their flexible outer covering called a tunic. When they colonize, the sea quirts can form mats made of thousands of individuals covering the ocean floor.

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So far, the creatures have been found at only one location in Cobscook Bay; further surveys will determine the full extent, Harris said.

For now, Harris doesn't believe there's an immediate concern in Cobscook Bay. But the creatures have caught the attention of state officials.

"Any new invasive species is a concern," said George Lapointe, commissioner of the Maine Department of Marine Resources in Augusta. "We share the concern of the impact on the ecology on the bay and the fisheries that are dependent on Cobscook."

On Georges Bank, a 2003 survey led by Page Valentine from the U.S. Geological Survey found sea squirt colonies over a 6-square-mile area. Last fall, a more intensive survey showed colonies over a 40-square mile area.

On Monday, a team from the U.S. Geological Survey and the National Oceanic and Atmospheric Administration was heading back to Georges Bank to try to determine whether the sea squirt infestation is continuing to grow.

Sea squirts are "very adaptable and persistent," said Terri Frady, a NOAA spokeswoman. "We don't really have any good ideas on what would contain it or control it."

Cobscook Bay is the center of marine activities for Washington County with salmon aquaculture operations, lobstering and clamming. Traditionally, scallops have been an important fishery, though their numbers have been low in recent years.

Nearly 20 biologists from around the country took part in an assessment of invasive creatures in the bay. The survey on Aug. 7-8 was organized by The Nature Conservancy and Cornell University with help from the Maine Sea Grant and the Cobscook Bay Resource Center.

Harris donned a wet suit and went searching for the sea squirts, which had been reported at the site of a failed oyster farm in the bay. Working in water that was only 15 feet deep at low tide, he found one colony the size of a dinner plate or a saucer for every square meter he surveyed.

Cobscook Bay represents the northernmost location on the East Coast where Didemnum have been confirmed. They've also been found near New Hampshire's Isle of Shoals and Portsmouth Harbor and are a problem around the world.

Harris believes the Cobscook site was contaminated by oysters brought from the Damariscotta River, the first confirmed location of sea squirts in New England waters.

Researchers believe Japanese oysters that were planted in the Damariscotta caused the original infestation years before they were discovered, Harris said.